A klystron tube is a special type of vacuum tube invented in 1937 by the Varian brothers. A vacuum tube is a piece of electronic equipment, usually a tubular glass structure, from which most of the air has been removed. The klystron tube was used for radar systems during the Second World War and is still in use for radar systems today. Oddly enough, during the Second World War, radar systems using this technology were favored by the Axis powers. The Allies primarily relied on another technology for building radar systems.
Microwave energy is produced by a klystron tube. In this application, it works in a similar manner to a pipe organ tube, except that the tube uses electrons, while the pipe organ tube uses air. When the air in the organ tube vibrates, the pipe organ tube emits sound energy of a specific frequency that we hear as a single note. When the electrons in the tube vibrate, it emits high frequency microwave energy that can be detected by a radar receiver.
Once the Second World War was over, Russell and Sigurd Varian continued research into microwave and radar energy with the intention of finding a cure for cancer. They founded Varian Medical Systems to commercialize this technology, and the company is still using technology based on the original klystron tube to develop leading edge cancer treatments.
The klystron tube has also made it into popular culture. The movies Forbidden Planet and My Stepmother was an Alien both make references to it. However, neither instance is an accurate depiction of its actual function. A great deal of research into the uses of this technology has been conducted at Stanford University, and their studies are still ongoing. It is worth a visit to the university's site if you wish to learn more about the technology behind the klystron tube.